Semiconductor wafers are tested prior to singulation into individual dice, to assess the electrical characteristics of the integrated circuits contained on the dice. A typical wafer-level test system includes a wafer handler for handling and positioning the wafers, a test controller for generating test signals, and a probe card for making temporary electrical connections with the wafer. In addition, a performance board associated with the probe card contains driver circuitry for transmitting the test signals to the probe card.
The test signals can include specific combinations of voltages and currents transmitted through the performance board and probe card to the wafer. During the test procedure response signals such as voltage, current and frequency can be analyzed and compared by the test controller to required values. The integrated circuits that do not meet specification can be marked or mapped in software. Following testing, defective circuits can be repaired by actuating fuses (or anti-fuses) to inactivate the defective circuitry and substitute redundant circuitry.
Different types of probe cards have been developed for probe testing semiconductor wafers. The most common type of probe card includes elongated needle probes adapted to electrically engage corresponding contacts on the wafer. An exemplary probe card having needle probes is described in U.S. Pat. No. 4,563,640 to Hasegawa et al. Another type of probe card includes buckle beam probes adapted to flex upon contact with the wafer. This type of probe card is described in U.S. Pat. No. 4,027,935 to Byrnes et al. Yet another type of probe card, referred to as a "membrane probe card", includes a membrane, such as polyimide, having contacts in the form of contact bumps thereon. An exemplary membrane probe card is described in U.S. Pat. No. 4,918,383 to Huff et al. Still another type of conventional probe card includes a silicon substrate and probe tips that have been micro machined and covered with a conductive layer. Such a probe card is described in U.S. Pat. No. 5,177,439 to Liu et al.
With any of the above types of probe cards, contacts on the probe card (e.g., probe needles, contact bumps, probe tips) must electrically engage contacts on the wafer (e.g., test pads, bond pads). A problem with making these electrical connections is that the electrical resistivity of the probe contacts can increase with continuous use of the probe card. Probe cards are designed to be used over extended periods of time with periodic cleaning and adjustments. However, a single probe card may test thousands of wafers prior to being cleaned and adjusted.
With extended use, the probe contacts can become covered with contaminants, such as particles, and residual photoresist from wafer fabrication processes. These contaminants tend to increase the electrical resistivity of the electrical connections between the probe contacts and the wafer contacts. The increased resistivity can increase the time required for test signals to be transmitted and received from the wafer. Also voltage and current values of the test signals can be adversely affected by the increased resistivity. The wafer contacts (e.g., aluminum bond pads) can also include layers that may affect the resistivity of the temporary electrical connections with the probe card, and the test signals transmitted through these connections.
In addition to contaminants which may affect resistivity, the metallic surfaces of the probe contacts and wafer contacts will typically be covered with a film of some type. Base metals such as copper, aluminum and nickel include a surface film comprising a metal oxide. This surface film can be up to several hundred Angstroms thick. Noble metals such as gold, can also include adsorbed gases, water vapor and organic molecules. The films are electrically insulative and can interfere with the free flow of electrons between the mating contacts.
It would be advantageous to be able to evaluate the electrical resistivity of the probe contacts and of the wafer contacts. In addition, it would be advantageous to be able to evaluate the contact resistance between the probe contacts and wafer contacts during a test procedure. These resistivity measurements could then be used during transmission and evaluation of test signals. This information could also be used to indicate cleaning of a probe card is required.
In view of the foregoing, the present invention is directed to an improved probe card that includes resistivity measuring circuitry, and to a method for fabricating the probe card.